1. Field of the Invention
The present invention relates generally to low emissivity (“low-E”) coatings, and more particularly to low-E coatings incorporating at least one metal oxide layer as the infrared (IR) reflecting layer(s).
2. Discussion of the Background
All United States patents and patent applications referred to herein are hereby incorporated by reference in their entireties. In the case of conflict, the present specification, including definitions, will control.
Low emissivity (low-E) coatings on glass are designed to permit the passage of visible light while reflecting and blocking emission of infrared (IR) radiation. High visible transmittance, low emissivity coatings on architectural windows, automobiles and commercial refrigerator and freezer doors can lead to substantial savings in costs associated with environmental control, such as heating and cooling costs.
Generally speaking, coatings that provide for high visible transmittance and low emissivity are made up of a stack, which typically includes a transparent substrate and an optical coating. The stack may include one or more thin metallic layers, with high IR reflectance and low transmissivity, disposed between anti-reflective dielectric layers. The anti-reflective dielectric layers are generally transparent materials selected to enhance visible transmittance. These systems reflect radiant heat and provide insulation from the cold as well as from solar radiation. Most low-E stacks in use today are based on metal layers, such as silver, sandwiched between transparent metal oxide dielectric layers. In general, the thickness of the dielectric layers are tuned to reduce inside and outside reflectance so that the light transmittance is high (>60%). The IR reflective metallic layers may be virtually any reflective metal, such as silver, copper, or gold. Silver (Ag) is most frequently used for this type of application due to its relatively neutral color.
However, while coatings incorporating sputter deposited Ag layers in combination with dielectric layers in multilayer stacks can provide high performance solar control products (i.e., close to neutral in both reflection and transmission), there also can be significant disadvantages in using silver layers in such low-E optical stacks.
First, suitable silver layers are not susceptible to on-line deposition methods in which the coating is applied to a hot glass ribbon as it is produced, i.e., before it is cut and removed from the production line, but are applied by off-line low pressure techniques, such as magnetron sputtering. This limitation of sputter deposited silver layers increases the final coated product production time. Second, such coatings have limited chemical and mechanical durability, requiring careful protection and handling during processing and shipping. Thin, transparent metal layers of Ag are susceptible to corrosion when they are brought into contact, under moist or wet conditions, with various corrosive agents, such as atmosphere-carried chlorides, sulfides, sulfur dioxide and the like. To protect the Ag layers, various barrier layers can be deposited on the Ag. However, the protection provided by conventional barrier layers is frequently inadequate. Thin, transparent metal layers of Ag are also susceptible to degradation upon heat treatment, bending and/or tempering. When coated glass is tempered or bent, the coating is heated along with the glass to temperatures on the order of 600° C. and above for periods of time up to several minutes. These thermal treatments can cause the optical properties of Ag coatings to irreversibly deteriorate. This deterioration can result from oxidation of the Ag by oxygen diffusing across layers above and below the Ag. The deterioration can also result from reaction of the Ag with alkaline ions, such as sodium (Na+), migrating from the glass. The diffusion of the oxygen or alkaline ions can be facilitated and amplified by the deterioration or structural modification of the dielectric layers above and below the Ag. Coatings on glass must be able to withstand such elevated temperatures. However, previously known multilayer coatings employing Ag as an infrared reflective film frequently cannot withstand such temperatures without some level of deterioration of the Ag film.
Thus, there remains a need for low-E coating stacks (and methods of making them) that overcome the various aforementioned problems known to those of skill in the art. In particular, there is a need for low-E optical coatings which exhibit retained or increased aesthetic appeal, and mechanical and/or chemical durability, and which can be tempered or heat strengthened, if desired. It would be desirable to have a coating which would provide a high performance solar control glazing without the disadvantages of the silver coatings referred to above, and which preferably would have a near neutral color in reflection and transmission.
Thin film low-E infrared reflecting coating layers based on tin oxide, or doped tin oxide, represent such an alternative that circumvents the various aforementioned problems that can occur with sputter coated Ag infrared reflecting layers.
Low-E tin oxide thin film coatings are well known. Such tin oxide coatings offer several advantages over sputter coated Ag infrared reflecting layers. One such advantage is that the tin oxide coatings can be pyrolytically deposited onto a surface of a heated glass ribbon. In other words, the tin oxide thin film layer can be pyrolytically deposited online, which reduces the production time of the final desired coated product. Another such advantage is that the pyrolytically deposited tin oxide thin film layers are hardcoats. Hardcoats generally have a higher degree of mechanical and chemical durability when compared to softcoats such as offline sputter deposited Ag thin film layers. Such hardcoats, when incorporated into a low-E optical stack, impart an increased resistance to degradation upon heat treatment, tempering or bending. However, tin oxide based low-E thin film layers generally do not possess the IR reflecting properties approaching those of metal, or Ag, based low-E thin film layers. In addition, such a pyrolytic coating on glass is heat resistant and the glass can be heat-strengthened or tempered without damaging the coating.
Thus, there remains a need in the art for metal oxide based low-E thin film layers that can overcome the above-noted problems associated with sputter deposited metal, or Ag, thin film layers. In particular, there remains a need in the art for metal oxide based low-E thin film layers that possess infrared reflecting properties approaching those of sputter deposited metal, or Ag, thin film layers.